Charged particle beam apparatuses have many functions, in a plurality of industrial fields, including, but not limited to, critical dimensioning of semiconductor devices during manufacturing, defect review of semiconductor devices during manufacturing, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring, testing and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
Thereby, some applications require a high resolution, e.g. of 10 nm or below, a large field of view and a high scanning speed. When a charged particle beam such as an electron beam is scanned over a flat surface, it is typically continuously refocused. The focal distance between the lens and the position to be imaged increases when the beam is deflected away from the axis (field curvature correction). In order to allow for high resolution and large field of view, the beam is therefore refocused during deflection. However, the application of a high scanning speed is an additional challenge.
For scanning the beam off the optical axis in order to achieve the required field of view, a magnetic or electrostatic deflector can be used. Additionally, due to the field curvature, correction of the focus length can be used. This has often been done by readjusting the magnetic field of the objective lens and/or by adjusting the beam energy in the objective lens. Another theoretical possibility would be to apply a common voltage to all electrodes in the microscope or to insert an additional electrode and changing potential of these electrodes or the additional electrode. However, it has been found that the latter would necessitate high voltages up to 1 kV for fields of view in the 100 μm area. Such high voltages are difficult to switch with the required speed and furthermore require electrical insulation which is difficult to achieve for these high voltages. Yet further, such electrodes represent relatively large capacitances, which are also difficult to switch quickly.
In view of the above, it is an object of the present invention to provide an improved retarding field scanning microscope using a charged particle beam, particularly an electron beam that would overcome at least some of the above problems.